This invention relates to bandpass filters and in particular to bandpass filters having variable bands of operation.
The prior art high frequency (HF) filtering designs required complex switching and aperture designs to cover broad operating frequency ranges. The prior art bandpass filters used in the HF band were designed to cover limited frequency ranges such as 3 to 1 or 4 to 1. Thus to cover a broad range of frequencies such as from 1.6 to 30 MHz required three filter designs. Other arrangements employ complex bandswitching schemes to selectively connect the coil comprising the resonators in series or in parallel. This prior art approach is limited to two bands and is also very costly; and the resulting filter usually provides non-optimal performance.